Prevention of bonding of aluminum powder metallurgy parts during sintering by separating the parts with oxidized particulate aluminum

ABSTRACT

Use of particles of aluminum having an oxide coating obtained by active oxidation thereof to prevent sticking together of aluminum powder metallurgy parts stacked or piled during sintering.

United States Patent [191 Brondyke et a].

[73] Assignee: Aluminum Company of America, Pittsburgh, Pa.

[22] Filed: June 26, 1972 [21] Appl. No.2 266,293

[52] U.S. Cl 75/223, 75/200, 264/58 [51] Int. Cl B22f 3/16 [58] Field of Search 264/58; 75/223, 200;

[ 1 Mar. 12, 1974 [56] References Cited UNITED STATES PATENTS 3,326,679 6/1967 Wallace 75/200 X FOREIGN PATENTS OR APPLICATIONS 836,566 3/1952 Germany 75/223 1,242,881 6/1967 Germany 75/223 Primary Examiner-Carl D. Quarforth Assistant ExaminerR. E. Schafer Attorney, Agent, or Firm-Abram W. Hatcher [57] ABSTRACT Use of particles of aluminum having an oxide coating obtained by active oxidation thereof to prevent sticking together of aluminum powder metallurgy parts stacked or piled during sintering.

7 Claims, N0 Drawings PREVENTION OF BONDING OF ALUMINUM POWDER METALLURGY PARTS DURING SINTERING BY SEPARATING THE PARTS WITH OXIDIZED PARTICULATE ALUMINUM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to sintering of aluminum powder metallurgy parts. More particularly, it relates to prevention of bonding of aluminum powder metallurgy parts during sintering. In the term aluminum, where not specifically identified otherwise, we include aluminum base alloys and mixtures of aluminum with other metallic orsimilar elements where at least 50 percent by weight aluminum is present.

2. Description of the Prior Art When sintering iron, copper, stainless steel and like powder metal parts, for example, by heating them continuously or batchwise in a sintering furnace which employs a reducing or neutral atmosphere or vacuum, it is known to stack them randomly in trays or on a conveyor belt to speed up production. Until recently this has not been practical for aluminum powder metal parts because of a tendency for the parts to bond together and become inseparable after the sintering operation. This may occur in the case of aluminum powder because the parts are generally sintered in a liquid phase which rapidly promotes alloying and bonding between individual powder particles as well as between adjacent parts.

Efforts to separate aluminum powder metallurgy parts by coating with materials which do not bond to or react with aluminum have met with only limited or partial success to data. For example, U. S. Pat. No. 3,326,679, which treats known aluminum powder compacting and sintering procedures in considerable detail, and the pertinent procedures of which are incorporated herein by reference, suggests employing powdered coating materials such as graphite, aluminum oxide, magnesium oxide, silica and molybdenum disulfide suspended in a liquid vehicle which is removed by heating the treated compact at a temperature below the sintering temperature to volatilize the liquid vehicle before sintering of the compact. In using such a system, however, it is necessary after sintering to remove the coating materials deposited from solution from the surface of the parts or compacts (compacted powder) by chemical, abrasive, or mechanical cleaning or the like.

SUMMARY OF THE INVENTION After extended investigation, we have found that these sticking and cleaning or residue problems may be substantially eliminated, often without the need for any surface treatment at all after the sintering step, by employing oxidized particulate aluminum as a material or medium to separate the parts. By oxidized particulate aluminum we mean particulate aluminum which has been actively oxidized, that is, has undergone some oxide coating of as much as 500 angstroms in averagethickness through contact with air atroomtemperature, is excluded from utility according to the invention as incapable of maintaining the desired substantially non-sticking separation of powder metallurgy parts during and after sintering thereof, the sintering normally being at a temperature of at least about 900F and more often at a temperature above about lO00F. In separating the parts according to the invention, sufficient particulate aluminum should be employed between them to keep them from touching one another. Otherwise, fusing together during sintering is likely. According to our invention the parts may be submerged in the oxidized aluminum particles during sintering. The oxidized particles of aluminum useful according to our invention are preferably of a size such that substantially none of them will pass through a mesh screen. Preferably they are of the -14 +28 size (U. S. Standard Sieve Series). Best results are obtained if the particles are not larger than three-eighths inch in their maximum dimension. I

The method of obtaining the oxide-coated aluminum particles useful according to our invention is not critical. The oxidation may be carried out, for example, by treating particulate aluminum with hot water (preferably at a temperature of at least l60F) which may contain. up to 1 percent of an hydroxyamine, for example, triethanolamine, or simply by oxidizing with oxygen or air at elevated temperature. The particulate aluminum to be oxidized may be prepared by any standard method of producing aluminum powder, for example, by molten metal atomization, grinding, precipitation in water or the like, as is well known to those skilled in the art.

An additional benefit gained by our method of using oxidized particulate aluminum to separate aluminum powder metallurgy parts during sintering is the reduction in the amount of staining caused from volatilized lubricant or from liquified lubricant which has been baked on the surfaces of the parts. While we do not wish to be bound by any particular theory as to why there is less staining according to the process of our invention, it appears that the lubricant may transfer to the particulate aluminum particles to thereby reduce the surface staining.

A further advantage of our invention is that in many instances superior tensile properties are obtained for the parts sintered when oxidized aluminum particles are used to separate them or to submerge them during the sintering thereof, for example, improvements in the ultimate tensile strength (UTS) and ultimate yield strength (UYS).

Additional advantages include (1) good temperature uniformity of covered parts or compacts because the aluminum particles or pellets conduct heat rapidly and (2) the fact that the particulate aluminum can be reused without being re-oxidized or re-treated prior to re-use.

DESCRIPTION or THE PREFERRED EMBODIMENTS For a more detailed description of our invention, reference is now made to the following examples, which are illustrative of the invention.

EXAMPLE 1 -8 mesh to +28 mesh particulate aluminum having a composition of 99.0% minimum aluminum content was oxidized in boiling distilled water containing 1 milliliter per liter of triethanolamine and placed between two 3-inch long by three-fourths inch wide bars of a compact (90% of theoretical density) of powdered aluminum base alloy of the following approximate composition by weight: 1% Mg, 0.6% Si, 0.25% Cu, balance aluminum. The oxidized particulate aluminum was used to separate the aluminum powder bars (compacts) stacked one upon another. Effective separation was achieved during sintering without the specimens sticking together and without any substantial amount of the particulate aluminum adhering to them. These parts were compared with similar parts sintered in a similar manner except for not being separated by particulate aluminum. The unseparated parts bonded or stuck together during sintering and became a cohesive mass.

EXAMPLE 2 In a second series of tests, alternate layers of oxidized particulate aluminum (99.0% minimum aluminum content), oxide layer thickness of approximately 2500 angstroms, particle size 14 +28, and aluminum powder metallurgy parts having the following approximate composition by weight, 1% Mg, 0.6% Si, 0.25% Cu, balance aluminum, were placed in a tray to a height of two inches. During sintering, by conducting through an oven heated to a temperature of l 150F, the specimens did not stick to one another or to the oxidized particulate aluminum. These were compared to similar specimens separated by unoxidized particulate aluminum, which bonded to the particulate aluminum and made the specimens difficult to separate.

EXAMPLE 3 The data in this example (see the following table) illustrate the higher tensile properties which result for parts submerged in, covered with, or separated according to the invention by oxidized particulate aluminum during sintering. The parts used in this test were an aluminum powder of the following approximate composition by weight, 1% Mg, 0.6% Si, 0.25% Cu, balance aluminum, compacted to 95% density and sintered 30 (a) oxide coating of approximately 2500 angstroms thickness; particulate in percent by weight 99.0% minimum aluminum content EXAMPLE 4 This example illustrates the comparative lack of criticality of the particle shape and particle size of the particulate aluminum used to separate aluminum powder metallurgy parts or compacts according to the invention. It also shows that according to our invention, unoxidized aluminum particles will not satisfactorily separate aluminum powder metallurgy parts during sintering.

Aluminum particles of the several shapes and sizes shown in the following table, both oxidized and unoxidized, were tested for non-sticking characteristics.

TABLE 2 Aluminum Powder aluminum base alloy of the following composition: 99.5% minimum aluminum content Aluminum powder compacts containing 1% by weight Mg, 0.6% by weight Si and 0.25% by weight Cu separated by the various sizes and shapes of particulate aluminum shown in the foregoing table were sintered for 30 minutes at l 150F in an oven and a nitrogen atmosphere having a 40F to 60F dew point. In each instance the compacts consisted of nine 3-inch long specimens and five 3 X 0.4 X 0.5-inch bars in two layers separated and covered with the test separation media shown in the table. The unoxidized powder sintered into an inseparable mass, that is, into a mass which was inseparable from the compacts between which it was placed. Boiling water-oxidized aluminum powder partially adhered to the aluminum powder parts (first item in the table) but was readily removable with sand paper after the sintering. In each instance the unoxidized particulate aluminum stuck to the parts, considerable force being required to separate the parts from the particulate aluminum. In contrast, in all cases where oxidized particulate aluminum was used, the parts did not stick and were easily removed from the media.

As a further comparison, several hundred automotive mirror mount buttons (powder compacts) of an aluminum base alloy containing the following approximate composition by weight, 4.4% Cu, 0.5% Mg, 0.8% Si, balance aluminum were covered with unoxidized particulate aluminum of the same composition as that specified above in this example for the oxide-coated particulate aluminum, and sintered 15 minutes at 1 F in nitrogen atmosphere having a -40F to 60F dew point. The pellets stuck to the specimens so severely that mechanical tumbling was required to achieve separation.

EXAMPLE 5 The mechanical properties of the specimens sintered with separating media as described in Example 4 are listed in the following table.

(l) as sintered; cooled at no controlled rate (2) heat treated 30 min. at 970F, cold water quenched, and aged 4 days'minimum at room temperature (3) heat treated 30 min. at 970F, cold water quenched, and aged 18 hr. at 320F.

EXAMPLE 6 The following table further illustrates that the aluminum particles useful according to the invention must be of the actively oxidized variety; The oxidized particulate aluminum used here had a minimum aluminum composition of 99% by weight.

TABLE 4 Oxidizing Average Oxide Effectiveness for Separating Treatment Thickness Parts during Sintering Unoxidized 500A poorsevere sticking Heated in Air 1200-1500A fairpartial sticking at 1000F Treated in 3000A excellentno sticking Boiling Water EXAMPLE 7 The following table illustrates representative ways in which different thicknesses of oxide coating of the oxidized particulate aluminum useful according to the invention may be produced.

TABLE Estimated Average Treatment Thickness A 10 min. in boiling water 2600 (no triethanolamine) 10 min. in boiling water 3400 (l ml/l triethanolamine) 5 min. in boiling water 700 (no triethunolamine) 5 min. in boiling water 2900 (l ml/l triethanolamine) 2 min. in boiling water 2800 (l ml/l triethanolamine) 10 min. in 160F water 2200 (l ml/l triethanolamine) 10 min. in 160F water 600 (no triethanolamine) 5 min. in 160F water 1700 (l ml/l triethanolamine) 2 min. in 160F water 1400 1 ml/l triethanolamine) While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Having thus described our invention and certain embodiments thereof, we claim:

1. In a process which comprises sintering aluminum powder compacts, the step which comprises during said sintering separating said compacts by oxidized particulate aluminum having an oxide coating thereon formed by treating aluminum powder with a hot aqueous liquid comprising hot water and being of a size such that substantially all the particles thereof are retained by a mesh screen, said separating being performed without first coating the compacts with a liquid dispersion of said oxidized particulate aluminum, thereby minimizing adhering of said compacts to one another during and after said sintering, said compacts after said sintering while separated by said oxidized particulate aluminum having higher tensile strength and yield strength than said compacts treated in the same manner but not separated by oxidized particulate aluminum during said sintering.

2. The step of claim 1 wherein the oxidized particulate aluminum has an oxide coating thereon of greater than about 500 angstroms in average thickness.

3. The step of claim 1 wherein the oxidized particulate aluminum has an oxide coating thereon of greater than about 1200 angstroms in average thickness.

4. The step of claim 1. wherein the particulate aluminum is of a size such that substantially all the particles thereof pass through a l4-mesh screen and substantially all of the particles thereof are retained by a 28- mesh screen.

5. The step of claim 1 wherein said compacts are heat treated after sintering and after sintering and heat treatment have higher tensile strength and yield strength than said compacts treated in the same manner but not separated by oxidized particulate aluminum during said sintering.

6. The step of claim 1 wherein the oxidized particulate aluminum has an oxide coating thereon formed by treating aluminum powder with hot water containing up to about 1% by weight hydroxylamine.

7. The step of claim 8 wherein the hydroxylamine is 

2. The step of claim 1 wherein the oxidized particulate aluminum has an oxide coating thereon of greater than about 500 angstroms in average thickness.
 3. The step of claim 1 wherein the oxidized particulate aluminum has an oxide coating thereon of greater than about 1200 angstroms in average thickness.
 4. The step of claim 1 wherein the particulate aluminum is of a size such that substantially all the particles thereof pass through a 14-mesh screen and substantially all of the particles thereof are retained by a 28-mesh screen.
 5. The step of claim 1 wherein said compacts are heat treated after sintering and after sintering and heat treatment have higher tensile strength and yield strength than said compacts treated in the same manner but not separated by oxidized particulate aluminum during said sintering.
 6. The step of claim 1 wherein the oxidized particulate aluminum has an oxide coating thereon formed by treating aluminum powder with hot water containing up to about 1% by weight hydroxylamine.
 7. The step of claim 8 wherein the hydroxylamine is triethanolamine. 